BREAKING: replace custom build scripts with nix

General:
- Switched to yarn to better work with nix-based tooling
- Switched package system to use nix. This stops double dependencies and slow cloud compile times, while providing more compile/runtime support to the Nix project
- Removed container builder in favor of internal container tooling
- Package versions no-longer need to be SemVer compliant
- Removed "piston package spec" files, replaced with nix-flake based runtimes
- Exported nosocket and piston-api as packages within the nix-flake
- Removed repo container
- Switched docker building to nix-based container outputting
- Removed docker compose as this is a single container
- Removed package commands from CLI

Packages:
- Move bash, clojure, cobol, node, python2, python3 to new format
- Remainder of packages still need to be moved

v2 API:
- Removed "version" specifier. To select specific versions, use the v3 api
- Removed "/package" endpoints as this doesn't work with the new nix-based system

v3 API:
- Duplicate of v2 API, except instead of passing in a language name an ID is used intead.

runtimes/python2.nix

@@ -0,0 +1,25 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.python2;
+in piston.mkRuntime {
+    language = "python2";
+    version = pkg.version;
+
+    aliases = [
+        "py2"
+    ];
+
+    run = ''
+    ${pkg}/bin/python2 "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.py" = ''
+                    print "OK"
+                '';
+            };
+        })
+    ];
+}